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(1) Field of the Invention
This invention relates to projection systems for microelectronics patterning, of the type having a mask leg, crossover with an aperture stop, and a substrate section, for projecting microelectronics patterns onto substrates, and particularly relates to a technique for maintaining the required numerical aperture and working distance, and for minimizing the lens diameter size requirements, by distributing optical elements respectively in mask section, crossover and substrate section, positioning the aperture stop to balance the optical path lengths of mask section to aperture stop and substrate section to aperture stop.
(2) Description of Related Art
The dramatic decrease in cost coupled with the equally dramatic increases in performance, throughput, yield and cost reduction in the electronics industry are attributable in large part to several innovations in the field of optical microlithography. Key factors like speed and performance of the chips, their associated packages and hence the computer systems are dictated by the minimum printable size offered by lithography.
A typical lithographic system includes exposure tool, mask, resist, and all of the processing steps to accomplish pattern transfer from a mask to a resist and then to devices.
Many types of exposure tools are currently used in the fabrication of electronic modules; these include contact and proximity printers, single-field projection imaging systems, step-and-repeat tools, scanning projection printers, and focused-beam direct-write systems. All existing lithography tools suffer from significant limitations, including one or more of the following disadvantages: defect generation on the substrate, mask life degradation, limited substrate size capability, limited resolution, low throughput, stitching errors, poor yield, high system cost, poor opto-mechanical performance, and inability to drill vias in batch mode.
Many of these disadvantages are minimized by prior art Anvik technique of overlapping hexagonal-seamless scanning of mask to substrate on a single-planar stage. This system configuration provides both high optical and scanning efficiencies, and combines high-resolution imaging with very large exposure area capability.
FIGS. 1 and 2xe2x80x94Single-Stage Seamless Scanning
FIG. 1 shows the invention in a context including much of the prior art Anvik technique as shown in perspective in FIG. 2. It uses the same complementary hexagonal overlapping scans and the same single-stage presentation of mask and substrate for locked alignment of mask to substrate in a large area patterning system. The major difference is that the prior art does not show the distributed configuration of the projection optics, nor the aperture stop physically positioned assymmetrically in the crossover to maintain optical symmetry in the projection system despite the optical path length increase inherent in the reverser.
FIG. 2 schematically illustrates a large-area, scan-and-repeat lithography system incorporating a prior art Anvik technique. The substrate and mask are fixedly mounted on the same platform of a single stage. The stage is capable of moving the substrate and mask in synchronism in both x- and y-directions. The illumination system, having an excimer laser light source, is such that its effective emission plane is in the shape of a regular hexagon, which is imaged by a condenser at the mask. A 1:1 projection lens images the pattern contained within the illuminated hexagonal region on the mask onto the substrate. A reversing unit ensures that the image is in the same orientation as the object.
The lens has a numerical aperture NA, determined by the required resolution. NA denotes the numerical aperture at the mask on the illumination side, and is determined by partial coherence considerations. The largest regular hexagon that can be inscribed within the circular image field of the lens is used as the corresponding exposure region on the substrate.
The single-planar stage allows the mask and substrate to scan in unison (say, along the x-axis) across their respective illumination regions to traverse the substrate length. Following a scan, the stage moves along y by an amount which we call the effective scan width (shown as w in FIG. 3). Now the substrate and mask are again scanned along x as before, after which they are again laterally moved along y, and the process is repeated until the entire substrate is exposed. The complementary overlap between adjacent hexagonal scans is such that the transition, from one scan to the next, is totally seamless and free from any exposure non-uniformity.
This high-throughput, high-resolution patterning in photoresists and via-generation capabilities, the Anvik lithography system technology, is highly attractive for patterning applications in high-volume fabrication of a broad range of large-format electronic products, including flat-panel displays, multi-chip modules, and printed circuit boards.
However, the important requirement of long conjugate lengths, to accommodate the reversing and folding devices in the path, have in the prior art limited the single-stage imaging system to moderate-NA optical subsystems with moderate field of view. The requirement of long conjugate distances severely impacts the design and the overall cost of an imaging system, especially while targeted toward systems with high-resolution and high-NA, by requiring very large, and thus very expensive, lenses.
Conventional imaging systems, using a dual-stage technique and separate mask and substrate scanning systems, differ in their working distances set by their conjugate distances.
In single-stage Anvik seamless scanning technology, the desire is to have large conjugate distances of the projection lens to comfortably accommodate the reverser and folding devices within the imaging system for single stage operation. This may not pose a serious challenge for moderate-NA projection lenses. Higher-NA projection lenses, if they were to have large conjugate distances, would make the lens element sizes very large. Large lens size would significantly affect the design time and effort to control some aperture-dependent aberrations like spherical aberration and coma. This cost increase is far from linear; large lenses increment cost over smaller lenses approximately by the cube as size doubles. There are other problems with increased lens size in the special lenses for high-fluence ultra-violet radiation such as is preferred for microelectronics image patterning. Consequently, this would substantially increase the cost of manufacturing and building such a huge lens system due to the inevitable tight tolerances. In the context of single-stage scanning imaging systems, any system with NAxe2x89xa70.25 may be considered as a high-NA system as the requirement of long conjugate length would start influencing the basic configuration of the imaging system. In addition, as mentioned earlier, the requirement of large conjugate length at high-NA can significantly affect the design and configuration of the projection lens.
History presented several configurations to projection systems that are widely used in the field of optical lithography. They are broadly classified as all-reflective, cata-dioptric and all-refractive types and the choice of their selection is truly guided by factors like material availability at operating wavelengths, resolution and depth of field (DOF) of the system, projection technique and cost of ownership. Single-stage scanning systems offer high potential for large-pattern lithography in future systems. However, the maximum NA and thus the minimum feature size of these single-stage scanning systems are usually limited as discussed above.
Single-stage 1:1 projection subsystems typically have a twice-folded optical path, with a mask section, crossover, and a substrate section. They offer seamless patterning of large microlithographic substrates, by overlapping complementary small-field scanning. A reverser compensates for optical reversal to maintain mask/substrate pattern congruence, but presents the problem of additional optical path length. A 1:1 projection lens, which must pass high-fluence laser light, adds another problem, high expense, particularly when desire for a larger field size and higher resolution suggests a larger numeric aperture (NA). The mask and substrate sections, even though vacant in prior art subsystems, have finite lengths to permit access to the scanning stage, and thus may add to the conjugate length.
The invention distributes the projection system over two partial lenses, in mask and substrate sections, and shortens the crossover, minimizing total optical path length, but locates the aperture stop in the crossover, remote from at least one of the distributed part-lenses. The aperture stop is positioned by means such as an auxiliary stage, physically asymmetrical between the two partial lenses, to balance the extra optical distance inherent in the reverser.
It is the object of the invention to minimize lens aberrations, size and cost, by distributing the projection system so as to permit significantly smaller-diameter lens elements and thus to minimize problems of cost, optical path length and manufacturing difficulty in a twice-folded optical projection system.
Another object of the invention to minimize the optical path length in a twice-folded projection system having a reverser.
Still another object of the invention is to provide working distance, including space for optical elements, without adversely extending conjugate distances.
A feature of the invention is the distributed location of the projection system as two part-lenses, located remote from each other in the vertical sections of the twice-folded projection system, with the aperture stop positioned physically asymmetrical but optically symmetrical to compensate for the added optical path length inherent in the reverser.
Another feature of the invention is the use of an auxiliary stage to relocate the aperture stop to compensate for the added optical path length inherent in the reverser.
An advantage of the invention is the optimized optical path length made possible by dividing the projection system into two distributed-part-lenses and locating them respectively in the vertical mask section and in the vertical substrate section.
Another advantage of the invention is the cost savings involved in having distributed-part-lenses rather than in having a single projection lens, each of the distributed part-lenses costing significantly less than half the cost of a single projection lens while providing other advantages including shortened optical path length and lowered system and replacement parts costs.
Still another advantage of the invention is that it continues to solve optical problems in projection patterning of microelectronics on large substrates, while permitting a diminished footprint in the extremely expensive environment of the clean room while at the same time lowering the cost of the projection system.
Other objects, features and advantages of the invention will be apparent to those skilled in the art, in light of the attached drawings and written description of a preferred embodiment.